1. Field of the Invention
This invention relates to an optical fiber amplifier to be used for an optical video distribution system or a wavelength-multiplexing communication system and, more particularly, it relates to an optical fiber amplifier with a high output level and a low noise level.
2. Prior Art
A source of excitation light with a wavelength of 980 nm or 1,480 nm is popularly used for an Er-doped type optical fiber amplifier (hereinafter simply referred to as optical fiber amplifier).
When a source of excitation light with the 980 nm wavelength is used for an optical fiber amplifier, the noise figure (NF) of the optical fiber amplifier will be 3 dB, which is very advantageous for the use of such an amplifier. However, when excitation light having the 980 nm wavelength is used, the efficiency with which the energy of excitation light is converted into that of signal light is as low as about 50%.
The conversion efficiency is inherently specific to the wavelength or the frequency of excitation light and can be obtained by calculation using formula (1) below. EQU Conversion efficiency=(Plank's constant.times.signal light frequency/Plank's constant.times.excitation light frequency)(1)
The value obtained by this formula is a theoretical limit value. Thus, the actual conversion efficiency will be smaller than the theoretical limit value and vary depending on the configuration of the optical fiber amplifier in question.
The noise figure that arises with the use of excitation light having the 980 nm wavelength was discussed in the lecture with Lecture No. TuB3 (Bulletin; pp. 134-137) at the Topical Meeting on Optical Amplifiers (August, 1990) held under the auspices of the Optical Society of America.
Some characteristic aspects of the conversion efficiency of optical fiber amplifiers were discussed in the lecture with Lecture No. MB3 (Bulletin; pp. 16-19) at the above identified international conference.
When a source of excitation light having the 1,480 nm wavelength is used for an optical fiber amplifier, the noise figure (NF) of the optical fiber amplifier will be between 6 dB and 9 dB, which is less advantageous if compared with a source of excitation light having the 980 nm wavelength. However, as may be seen from the above formula, when signal light has a wavelength of 1,530 nm, the conversion efficiency is as high as between 70% and 85% because of the proximity of the wavelength of excitation light and that of signal light.
The noise figure that arises with the use of excitation light having the 1,480 nm wavelength was discussed in the lecture with Lecture No. MD7 (Bulletin; pp. 68-71) also at the above identified international meeting. Additionally, some characteristic aspects of the conversion efficiency of optical fiber amplifiers using excitation light with the 1,480 nm wavelength were discussed in the lecture with Lecture No. TuC3 (Bulletin; pp. 156-159) at the same international conference.
Meanwhile, in the course of technological development of optical telecommunications networks in recent years, there has been an increased demand for optical fiber amplifiers with a high-output level and a low-noise level that are adapted to multipoint connection and wavelength-multiplexing communication.
Optical fiber amplifiers that can meet the demand are required to show an optical output level of more than +20 dBm, while conventional optical fiber amplifiers commonly have an output level between +16 dBm and +19 dBm.
Japanese Patent Application Laid-Open No. 4-149525 discloses an optical fiber amplifier combining the low noise characteristic of excitation light with the 980 nm wavelength and the high-output characteristic of excitation light with 1,480 nm wavelength. FIG. 6 of the accompanying drawings schematically illustrates an optical fiber amplifier 60 disclosed in Japanese Patent Application Laid-Open No. 4-149525. A source of excitation light 62 with the 980 nm wavelength is arranged at the input side of the amplifying optical fiber 61 while another source of excitation light 63 with the 1,480 nm wavelength is arranged at the output side of the amplifying optical fiber 61. In FIG. 6, reference numeral 64 denotes an optical coupler.
A similar optical fiber amplifier is proposed in U.S. Pat. No. 5,140,456.
FIG. 7(a) is a graph showing the output characteristic of an optical fiber amplifier as shown in FIG. 6 and FIG. 7(b) is a graph showing the noise characteristic of the same optical fiber amplifier of FIG. 6. The excitation light source 62 with the 980 nm wavelength has an output level of 90 mW and the excitation light source 63 with the 1,480 nm wavelength has an output level of 140 mW both in terms the optical fiber pig tail of an excitation LD module, representing the currently available highest practical output level of a stand-alone excitation LD module. As seen from FIGS. 7(a) and 7(b), while the optical fiber amplifier shows a good noise characteristic, its output is +19 dBm to +20 dBm at most.
FIG. 8 shows a two-stage optical fiber amplifier 70 designed to raise the output level, keeping a low noise performance.
Referring to FIG. 8, there are provided a front stage excitation light source 71, back stage excitation light sources 72 and 73, a front stage amplifying optical fiber 74 and a back stage amplifying optical fiber 75.
The front stage optical fiber amplifier 70A has the wavelength of 980 nm and operates for excitation, maintaining its low-noise characteristic, whereas the back stage optical fiber amplifier 70B has the wavelength of 1,480 nm and is adapted to bidirectional excitation by the excitation light sources 72 and 73 in order to realize a high output level.
With such bidirectional excitation using the wavelength of 1,480 nm, the amplifying optical fiber 75 has to have a large length in order to provide a high output level by absorbing excitation light and maintaining a sufficiently high conversion efficiency. On the other hand, when the back stage excitation light amplifier 70B has a long amplifying optical fiber 75 in the two-stage optical fiber amplifier 70, the backwardly advancing spontaneously emitted light (hereinafter referred to as backward amplified spontaneous emission or backward ASE) coming from the back stage amplifier operates to saturate the gain of the front stage amplifying optical fiber 74 and consequently reduce that of the front stage optical fiber amplifier 70A. The overall noise figure (NF) of a two-stage optical fiber amplifier is expressed by formula (2) below. EQU overall NF=front stage NF+{(back stage NF-1/front stage gain}(2)
It will be seen that the NF of the back stage becomes unnegligible to degrade the overall NF if the front stage has a small gain so that some means has to be provided to remove or reduce the adverse effect of the backward ASE. Techniques proposed for this purpose include the following.
(1) Insertion of an optical isolator between the front stage and the back stage; e.g., a method disclosed in U.S. Pat. No. 5,233,463.
(2) Insertion of an optical filter between the front stage and the back stage to remove ASE of the 1,530 nm band; e.g., a method disclosed in U.S. Pat. No. 5,406,411.
(3) The use of a relatively short amplifying optical fiber in the back stage to minimize the ASE of the back stage.
However, with any or all of the above three methods, the gain of the front stage is apt to be saturated and reduced to consequently degrade the overall NF in an optical fiber amplifier adapted to an optical video distribution system or a wavelength multiplexing communication system because of the high input level of signal light.
Additionally, if the ASE minimization technique of (3) above is used in a two-stage optical fiber amplifier having a configuration as shown in FIG. 8, there arises a problem that the energy of excitation light with the 1,480 nm wavelength is not satisfactorily absorbed by the amplifying optical fiber 75 and any residual excitation energy will be simply wasted to result in a poor output performance as illustrated in FIG. 7(b).
Still additionally, an excitation light source is required to continously oscillate with an energy output level of 100 mW or more on a highly reliable basis. However, it is known that excitation light sources with the 980 nm wavelength can abruptly suspend the optical operation.
Thus, in order for an optical system comprising an optical fiber amplifier to improve its reliability, the optical fiber amplifier is required to maintain its optical output characteristic above a predetermined level if its excitation light source with the 980 nm wavelength abruptly suspends its optical operation possibly by means of an additional excitation light source it comprises.
Referring to the optical fiber amplifier of FIG. 8, if the excitation light source with the 980 nm wavelength suspends its optical operation, the front stage amplifying optical fiber 74 operates as a light absorbing medium for any signal light to consequently degrade the noise figure and reduce the optical power output level of the optical fiber amplifier.
In view of the above described circumstances, it is therefore the object of the present invention to provide an optical fiber amplifier that can maintain its optical power output above a level exceeding +20 dBm and a low noise figure if its excitation light source with the 980 nm wavelength abruptly suspends its operation.